1. Field of the Invention
The present invention relates to a photoresist film and a method for formation of a pattern for the photoresist film.
2. Description of the Related Art
Large-scale integrated semiconductor circuits (hereafter referred to as "LSIs") are becoming more and more highly integrated in recent years, requiring scale reduction to a size smaller than the design rule. As a light source to be used in a photolithography step, a mercury lamp (i-beam; 365 nm) or a KrF excimer laser (248 nm) is becoming prevalent. However, a design rule to be expected in the future (0.2 .mu.m or less) requires formation of a pattern of a size less than the wavelengths of these light sources, so that a new method must be developed.
An approach to this demand using a mono-layer resist film is now being studied along the following line. Namely, a method of using the deformation radiation or phase shift method in combination with the KrF excimer laser is now studied as the first method. This method achieves a resolution of 0.14 .mu.m with a film thickness of 1000 .ANG.. Also, a method of adopting an ArF excimer laser (193 nm) as a new light source is now being studied as the second method. The ArF excimer laser achieves a resolution of 0.12 .mu.m with a film thickness of 1000 .ANG.. For use in the future, a method of using the deformation radiation or phase shift method in combination with the ArF excimer laser is studied as the third method. This method achieves a resolution of 0.1 .mu.m with a film thickness of 1000 .ANG..
When these methods are used, it is possible to form a very fine pattern with a resolution of 0.15 .mu.m or less if the film thickness is not considered. Actually, however, an etching step must be carried out after the photoresist film is patterned, so that the thickness of the photoresist film must be at least 5000 .ANG.. The resolution for this film thickness of 5000 .ANG. is 0.18 .mu.m by the first method, 0.16 .mu.m by the second method, and 0.14 .mu.m by the third method. Therefore, according as the thickness of the photoresist film increases, the resolution becomes worse than the original resolution (achieved by the thickness of 1000 .ANG.) by about 0.04 .mu.m. In other words, as long as a pattern is formed using a conventional mono-layer photoresist film, the thickness of the photoresist film is determined by taking account of the resistance to etching in the next step, so that the advantage of fine resolution is not sufficiently drawn out yet.
On the other hand, a patterning method known as the surface modification method is studied, in which the surface of a photoresist film is silylated after the exposure to light, and then a dry development is carried out to form a pattern having a high resolution and excellent resistance to dry etching. According to this technique, an initial pattern is formed in a region of about 1000 .ANG. thickness within the surface of the photoresist film, so that the resolution is higher than the one achieved by the conventional methods. Also, since the silylated layer formed in the surface can firmly protect the lower layer, the pattern can be made to have an excellent resistance to dry etching. However, at the dry development step, a plasma cuts a sidewall of the photoresist layer under the silylated layer, thereby forming a tapered cross section. Also, because of an inherent property that the formation of the silylated surface proceeds by diffusion within the substance, the stability of the pattern dimension is poor, so that the method is hardly practicable.
Accordingly, a patterning method using a multi-layer photoresist is now being developed. For example, in a method disclosed in Japanese Unexamined Patent Publication No. Hei 7(1995)-142365, a lower resist layer 43 which is sensitive to the i-beam is formed on a film 42 to be etched in a later step, and an upper resist layer 44 which is sensitive to the KrF excimer laser beam is mounted on the lower resist layer 43 (FIG. 5(a)). Subsequently, a KrF excimer laser beam is applied using a mask, followed by development to pattern only the upper resist layer 44 (FIG. 5(b)). Then, the i-beam is applied over an entire surface, followed by development. However, although a pattern can be formed in principle by this method, the applied beam cannot be completely shielded by the upper resist layer 44 at the second exposure step using the i-beam, so that as a result the lower resist layer 43 as a whole is exposed to the i-beam, thus making a line width of the lower resist layer 43 non-uniform and making it difficult to form a good pattern, as shown in FIG. 5(c). Here, in FIGS. 5(a) to 5(c), the reference numeral 41 represents a wafer substrate.
Therefore, at present, a photoresist film that overcomes the problem of decrease in the resolution due to the thickness of the photoresist film has not been developed yet.